Merge pull request vega#703 from vega/v3.0.0-beta.12

v3.0.0-beta.12

Release v2.6.5.

Update PORTING_GUIDE.md.

Release v2.6.4.

Fix beta version number.

Merge pull request vega#674 from vega/beta.8

Version 3.0.0-beta.8

Merge pull request vega#672 from vega/beta.7

Vega 3.0.0-beta.7

Update Travis CI config.

Bump beta version number.

Bump beta version number.