update go 1.5beta1 -> go 1.5beta2

See https://groups.google.com/forum/#!topic/gonum-dev/GKJjAtDxR9I for discussion

Author: jonlawlor

.travis.yml

@@ -2,7 +2,7 @@ language: go
 
 # Versions of go that are explicitly supported by gonum.
 go:
- - 1.5beta1
+ - 1.5beta2
  - 1.3.3
  - 1.4.2