Simplify Travis build matrix

Make .travis.yml more readable (and function better for alt. rubies),
at the expense of more some unwanted dependencies being installed.

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Revert "Make my sql2 Bundler group name on JRuby match MRI"

This reverts commit b802c1275325a5b685c7e14bbd57926cc7e0418f.
Update Migration utils to only require syck on MRI

Signed-off-by: Alex Coles <alex@alexbcoles.com>