travis: always 'go get -u' in 'before_install'

See https://github.com/dominikh/go-tools/issues/76#issuecomment-288189194.

Signed-off-by: Gyu-Ho Lee <gyuhox@gmail.com>

.travis.yml

@@ -41,10 +41,10 @@ addons:
     - libhunspell-dev
 
 before_install:
- - go get -v github.com/chzchzchz/goword
- - go get -v honnef.co/go/tools/cmd/gosimple
- - go get -v honnef.co/go/tools/cmd/unused
- - go get -v honnef.co/go/tools/cmd/staticcheck
+ - go get -v -u github.com/chzchzchz/goword
+ - go get -v -u honnef.co/go/tools/cmd/gosimple
+ - go get -v -u honnef.co/go/tools/cmd/unused
+ - go get -v -u honnef.co/go/tools/cmd/staticcheck
 
 # disable godep restore override
 install: