Update CHANGELOG.md

Release opencensus-ext-stackdriver v0.8.0 (release commit) (census-in…

…strumentation#1063)

Release 0.7.13 (census-instrumentation#1032)

Release for v0.7.12 (census-instrumentation#995)

Release for v0.7.11 (census-instrumentation#960)

Release for v0.7.10 (census-instrumentation#926)

Update version

Release for v0.7.8 (census-instrumentation#912)

Release for v0.7.7 (census-instrumentation#853)

Bump version for release to 0.7.6 (census-instrumentation#819)