Only test against 1.5 and up

Go 1.5 is the first one which supports the variable replacement we're
using for the flags. Any older versions aren't supported by the Go team
now that 1.7 is out, so you should be on one of these versions.

.travis.yml

@@ -5,10 +5,6 @@ sudo: required
 install: ./script/install-libgit2.sh
 
 go:
-  - 1.1
-  - 1.2
-  - 1.3
-  - 1.4
   - 1.5
   - 1.6
   - 1.7