Prepare for 0.4.0 release (census-instrumentation#604)

Release opencensus-0.4.0 and multiple new versions of ext packages. This
release includes breaking changes related to the context API and stats/metrics
exporters. It also adds a dependency on the new opencensus-context package.

Merge branch 'v0.3.1-ext-release' into v0.3.x

Prepare for 0.3.0 release (census-instrumentation#554)

Prepare for 0.2.0 release

Bump version to 0.1.11

and add a changelog.

Bump version to 0.1.10

Pick up census-instrumentation#369 to replace hardcoded version with __version__ file and bump
the version number.

Build only vX.Y.Z tags, only from release branches (census-instrument…

…ation#413)

Prepare for 0.1.8 release (census-instrumentation#339)

Prepare for opencensus 0.1.5 release (census-instrumentation#178)

Prepare opencensus0.1.4 release (census-instrumentation#148)