oauth2: don't require the appengine package during go-get

"appengine appenginevm" requires the Classic dependencies to
be go-getted. Our travis build doesn't have the right setup
to checkout the Classic App Engine package to the Travis
instance.

Change-Id: I13daa505be4c8ac209021c7e69a114f5eddd6d1e
Reviewed-on: https://go-review.googlesource.com/2982
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

.travis.yml

@@ -8,7 +8,7 @@ install:
   - export GOPATH="$HOME/gopath"
   - mkdir -p "$GOPATH/src/golang.org/x"
   - mv "$TRAVIS_BUILD_DIR" "$GOPATH/src/golang.org/x/oauth2"
-  - go get -v -t -d -tags='appengine appenginevm' golang.org/x/oauth2/...
+  - go get -v -t -d golang.org/x/oauth2/...
 
 script:
   - go test -v golang.org/x/oauth2/...