Updating google app engine sdk versions from 1.9.17 to 1.9.21

[jamesjnadeau]

The runtime setting for php has changed and was causing me issues, mostly complaints about it validating php vs php55. This should fix the issues I was having, and probably others.

See release notes: https://code.google.com/p/googleappengine/wiki/SdkReleaseNotes

[ksimuk]

thank you. merged