Fix build tags when including a repository and tag name

When a tag is appended to the repository as in 
```
sti build https://github.com/csrwng/simple-ruby.git openshift/ruby-20-centos test/image:001
```
The resulting image repository is named test/image:001 (as opposed to just test/image with a tag 001)
```
REPOSITORY                                         TAG                   IMAGE ID            CREATED             VIRTUAL SIZE
test/image:001                                     latest                68075c4924da        6 minutes ago       447.7 MB
```

Expected:
```
REPOSITORY                                         TAG                   IMAGE ID            CREATED             VIRTUAL SIZE
test/image                                         001                   68075c4924da        6 minutes ago       447.7 MB
```