Replace vendor pngs with svg export

For consistencies sake, replaces pngs with exports from the related
svg versions. This solves some inconsistencies in coloring from the
previous pngs.
Change 'GPA' to 'code climate' (fixes olivierlacan/shields#7)
Change green used in '4.0' badge to '3.0' green

Considering the 40 badges involved for code climate (0.0 to 4.0),
4.0 having a special green would lead to only one badge having
this shade, or possibly 5 (3.5-4.0). Instead, changing 4.0 to match
3.0's green will make more sense and keep things consistent.
Merge pull request #8 from ackerdev/master

Replace vendor PNGs with SVG export versions