Front-matter should be at start of file

It's the theme of the moment ; regexp checking.

Just in case we have two line start with --- in the file, we want to
make sure it's not interpreted as a front-matter.
make yaml parsing errors more informative
Update redcarpet support to version 2
Better error reporting on Liquid exceptions
Update pygments.rb version
Only pass first class to Pygments

This prevents an exception if something like the following is used:

~~~ {foo bar}
some code
~~~
Add license to gemspec
Merge pull request #683 from petedmarsh/master

Add license to gemspec
Merge branch 'redcarpet-update' of https://github.com/tombell/jekyll into tombell-redcarpet-update
Update history.
Remove newline to fix failing Redcarpet test.
Merge branch 'master' of https://github.com/wbrady/jekyll into wbrady-master
Update history.
Merge branch 'master' of https://github.com/Neil-Smithline/jekyll into Neil-Smithline-master
Clean up default config spec.
Merge branch 'fix/front-matter-regexp' of https://github.com/zimbatm/jekyll into zimbatm-fix/front-matter-regexp
Update history.