Don't use solarized theme unless in GVIM

Use solarized theme only if running GVIM. If using VIM on a
terminal, use old github theme. Most of the time I run vim
on a graphical environment I use GVIM. I don't run VIM from an
X terminal. I'd only use vim from a terminal if I'm running a
TTY session (Ctrl+Alt+F1..F6), which is actually often the case.
When on TTY, Solarized theme looks ugly and colors are kinda
unreadable.