Switch to "serve" verb.

The grunt community has moved to using verbs consistently in their
task names, so "server" has been renamed "serve"; getting it wrong
results in a bizarro error, so this is slightly nicer for newcomers
to the project with history in other grunt projects.

Change-Id: If1266371d0e548e71f579e440ba7d4e7db07918f
Merge "Switch to "serve" verb."