Support -Pspace=<...> vs. -Pproduction / -Pstaging

Prior to this commit, deployment required specifying

    ./gradlew deploy -Pproduction # OR
    ./gradlew deploy -Pstaging

This worked fine, but required unnecessarily complex conditional logic
in deploy.gradle. This change refactors this logic such that -Pspace is
required and the value is treated as the target space. 'production' is
treated as a reserved value and gets special treatment, e.g.:

 - 'panda' database instead of default 'hippo'
 - 'spring.io' and 'www.spring.io' URIs are mapped, instead of the
   default '${space}.spring.io'

If the value of the space property is anything other than 'production',
then the defaults mentioned above apply instead.

If the space property is not present and the build is running on Travis
CI, the implementation now looks to the name of the current git branch
to determine the target space. This allows us to push changes to master
in order to deploy to production, and to push branches named stage-foo,
stage-bar, etc to deploy to staging.

See also #331
Merge pull request #337 from spring-io/space

Support -Pspace=<...> vs. -Pproduction / -Pstaging