Add rake task to list version stats.

I wanted a way to get some fun stats to put in the 3.0.0 final release blog post and here's what I came up with.  Running `rake "version_stats[v2.14.0...v2.99.0.rc1|v2.14.0...v3.0.0.rc1]"` produces:


### Combined: 

117 contributors: Aaron Blew, Aaron Kromer, AbuSabah, Adam Farhi, Adarsh Pandit, Adrian CB, Adrian Gonzalez, Alex Peattie, Alex Rothenberg, Alex Yaremyuk, Alexander Clark, Alexey Fedorov, Alexey Pisarenko, Andy Henson, Andy Lindeman, Andy Waite, Arthur Neves, Arthur Nogueira Neves, Ashish Dixit, Ben Hamill, Ben Hoskings, Ben Moss, Ben Orenstein, Billy, Bradley Schaefer, Brandon Turner, Brian Fontenot, Christof, Claudio B., Damian Galarza, Daniel Fone, Daniel Murphy, Darryl Pogue, David Chelimsky, David Long, Eloy Espinaco, Erik Michaels-Ober, Federico Ravasio, Florian Thomas, Fujimura Daisuke, Giovanni Cappellotto, Giuseppe Capizzi, Grant Hollingworth, Guilherme Carvalho, Hendy Tanata, Hugo Baraúna, Ivo Wever, Jakub Racek, Jared Norman, Jay Hayes, Jeff Wallace, John Feminella, John Firebaugh, John Voloski, Jon Rowe, Jonathan del Strother, Josef Šimánek, Justin Coyne, Karthik Kastury, Katsuhiko Nishimra, Keiji, Kelly Stannard, Kenrick Chien, Lucas Mazza, Marc-Andre Lafortune, MasterLambaster, Matijs van Zuijlen, Matt Sanders, Matthew M. Boedicker, Mauricio Linhares, Michael Gee, Michael de Silva, Michi Huber, Myron Marston, Nerian, Nick DeLuca, Olle Jonsson, Oren Dobzinski, Paavo Leinonen, Parker Selbert, Paul Annesley, Pedro Gimenez, Pete Higgins, Peter Alfvin, Postmodern, Prathamesh Sonpatki, Puneet Goyal, René Föhring, Reyes Yang, Rodrigo Rosenfeld Rosas, Ryo Nakamura, Salimane Adjao Moustapha, Sam Phippen, Sergey Pchelincev, Stephen Best, Steve Richert, Stuart Hicks, Tay Ray Chuan, Thijs Wouters, Thomas Drake-Brockman, Thomas Holmes, Thomas Stratmann, Tim Cowlishaw, Tim Watson, Tom Stuart, Travis Herrick, Vinicius Horewicz, Vipul A M, Xavier Shay, Yoshimi, Yuji Nakayama, lucapette, maxlinc, modocache, sanemat, thepoho, vanstee

Total Commits: 3942

Merged pull requests: 845

### rspec-expectations: 

41 contributors: Aaron Kromer, Adam Farhi, Adrian Gonzalez, Alexey Pisarenko, Andy Henson, Andy Lindeman, Ben Moss, Ben Orenstein, Bradley Schaefer, Brandon Turner, Claudio B., Damian Galarza, Daniel Fone, Daniel Murphy, David Chelimsky, Eloy Espinaco, Erik Michaels-Ober, Federico Ravasio, Hendy Tanata, Hugo Baraúna, Ivo Wever, Jared Norman, Jeff Wallace, John Voloski, Jon Rowe, Katsuhiko Nishimra, Kenrick Chien, Myron Marston, Nerian, Pedro Gimenez, Pete Higgins, Prathamesh Sonpatki, René Föhring, Reyes Yang, Sam Phippen, Thijs Wouters, Thomas Holmes, Xavier Shay, Yuji Nakayama, lucapette, modocache

Total Commits: 911

Merged pull requests: 188

### rspec-mocks: 

33 contributors: Aaron Kromer, Adarsh Pandit, Andy Lindeman, Arthur Nogueira Neves, Ashish Dixit, Ben Moss, Bradley Schaefer, David Chelimsky, Federico Ravasio, Grant Hollingworth, Guilherme Carvalho, Jon Rowe, Jonathan del Strother, Justin Coyne, Marc-Andre Lafortune, Mauricio Linhares, Michael Gee, Michi Huber, Myron Marston, Nick DeLuca, Oren Dobzinski, Paul Annesley, René Föhring, Sam Phippen, Stephen Best, Stuart Hicks, Thomas Holmes, Tim Cowlishaw, Vinicius Horewicz, Xavier Shay, Yuji Nakayama, maxlinc, sanemat

Total Commits: 1002

Merged pull requests: 239

### rspec-core: 

46 contributors: Aaron Kromer, Adam Farhi, Adrian CB, Alex Peattie, Alexander Clark, Alexey Fedorov, Andy Lindeman, Arthur Neves, Ashish Dixit, Ben Hoskings, Ben Moss, Bradley Schaefer, David Chelimsky, Federico Ravasio, Giuseppe Capizzi, Jay Hayes, John Feminella, Jon Rowe, Karthik Kastury, Keiji, Kelly Stannard, MasterLambaster, Matijs van Zuijlen, Matthew M. Boedicker, Michael de Silva, Michi Huber, Myron Marston, Nerian, Parker Selbert, Pete Higgins, Peter Alfvin, Postmodern, René Föhring, Ryo Nakamura, Sam Phippen, Sergey Pchelincev, Steve Richert, Tay Ray Chuan, Thomas Stratmann, Tom Stuart, Travis Herrick, Vipul A M, Xavier Shay, Yoshimi, Yuji Nakayama, thepoho

Total Commits: 1551

Merged pull requests: 331

### rspec: 

5 contributors: Aaron Kromer, Andy Lindeman, Jon Rowe, Josef Šimánek, Myron Marston

Total Commits: 21

Merged pull requests: 3

### rspec-rails: 

37 contributors: Aaron Blew, Aaron Kromer, AbuSabah, Alex Rothenberg, Alex Yaremyuk, Andy Lindeman, Andy Waite, Ben Hamill, Billy, Bradley Schaefer, Brian Fontenot, Christof, Darryl Pogue, David Long, Florian Thomas, Fujimura Daisuke, Giovanni Cappellotto, Jakub Racek, John Firebaugh, John Voloski, Jon Rowe, Lucas Mazza, Matt Sanders, Myron Marston, Olle Jonsson, Paavo Leinonen, Puneet Goyal, René Föhring, Rodrigo Rosenfeld Rosas, Salimane Adjao Moustapha, Sam Phippen, Thomas Drake-Brockman, Thomas Holmes, Tim Watson, Xavier Shay, Yuji Nakayama, vanstee

Total Commits: 457

Merged pull requests: 84


<hr>

Any other stats folks want to see?