Don't use Pathname#/ in performance-critical code

This method (well, really, #join) is *twice* as slow as simple
concatenation, and shouldn't really be used at all in non-Formula code.
Avoid slow operations in FormulaPin#initialize

A FormulaPin object is created every time Formula is instantiated, so
don't do filesystem operations or Pathname concatenation eagerly.
Performance fix for Pathname#chop_basename

This is an internal method, but is called a bunch of times in
performance-critical codepaths, and is ultra slow because the constant
is interpoplated into the Regexp each time the method is called.

Alas, this has been fixed in Ruby 1.9+.
Performance fix for Pathname#prepend_prefix

See 05a456c231dc6da7cb0f7c70cb21feaf9a0d803c; same story.
Style nit
Avoid repeated interpolation here too
jenkins 1.511

Closes #19206.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
mpfr 3.1.2

Closes #19162.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>
mpfr 3.1.2 bottles.
vim 7.3.896

Closes #19211.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
heroku-toolbelt 2.37.2

Closes #19207.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
spiped 1.3.1

Closes #19201.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
rtorrent 0.9.3

Closes #19199.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
libtorrent 0.13.3

Closes #19198.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
gdk-pixbuf 2.28.1
gtk+3 3.8.1
gtkmm 2.24.3
gtkmm3 3.6.0
Clean up remaining DownloadStrategy initializers
Avoid expensive Pathname concatenation