build: fix docs-content branch publish (#12690)
build: fix docs-content branch publish (#12690)

Currently the docs-content output will be overwritten each time a Travis job runs in another branch (e.g. `master`, `6.4.x`, `6.x`). This is because the given branches cannot be created in the `material2-docs-content´ repository.

This is not good because if we want to release a new patch version, we have no way to properly pull the docs-content data that matches the given new version. Meaning that the docs-content and the `@angular/material-examples` package might be already updated for the next `major` version and we run into runtime exceptions due to the breaking changes.
Jefiozie/material2,josephperrott/material2,jelbourn/material2,andrewseguin/material2,mmalerba/material2,angular/components,DevVersion/material2,crisbeto/material2,mmalerba/material2,mmalerba/material2,angular/components,andrewseguin/material2,jelbourn/material2,mmalerba/material2,crisbeto/material2,DevVersion/material2,Jefiozie/material2,josephperrott/material2,Jefiozie/material2,DevVersion/material2,DevVersion/material2,josephperrott/material2,angular/components,josephperrott/material2,crisbeto/material2,Jefiozie/material2,DevVersion/material2,crisbeto/material2,jelbourn/material2,andrewseguin/material2,andrewseguin/material2,josephperrott/material2,Jefiozie/material2,andrewseguin/material2,angular/components,jelbourn/material2,mmalerba/material2,jelbourn/material2