oauth2: make sure travis runs at the target commit, not head
oauth2: make sure travis runs at the target commit, not head

Also, change the target Go version to 1.4 and make the 'go get' command
only perform the download step (no building).

Change-Id: Iafa3c218d2418b2a2681ce2c03bd1ff833afb317
Reviewed-on: https://go-review.googlesource.com/1702
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Reviewed-by: Burcu Dogan <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>

golang/oauth2,broady/oauth2,awormuth/oauth2,flowlo/oauth2,dsriram/oauth2,zj8487/oauth2,walmartlabs/oauth2,hoysoft/oauth2,equinux/oauth2,hoonmin/oauth2,wmydz1/oauth2,djalal/oauth2,freeformz/oauth2,jgalilee/oauth2,cw2018/oauth2,drewwells/oauth2,qtzheng/oauth2,steveruckdashel/oauth2,pengwynn/oauth2-1,Kuzmin/oauth2,heshed/oauth2,helojo/oauth2,EagerIO/oauth2,cllu/oauth2,jboeuf/oauth2,tpiha/oauth2,danielkarch/oauth2,dbmedialab/golang-oauth2,lancecarlson/oauth2,paul-at-start/oauth2