travis: add 1.7
travis: add 1.7

1.7 was released on 2016/08/15.
https://golang.org/doc/go1.7

Our current policy is to test up to ten build entries in Travis. This
puts us at 9, so 1.8 will bump off 1.4.3.

1.5.4 is added to the allowed failures now, since our policy says to
only consider failures for the two most recent; that'd be 1.7 and
1.6.3.

leenipper/xgo,exercism/xgo,robphoenix/exercism-go,leenipper/xgo,exercism/xgo,robphoenix/exercism-go