Added ability to switch version in documentation website

Website header now contains version number which is a link.
Clicking the link redirects you to a page that lists all the previous versions of the website.

Considering that we do a branch cut every 2 weeks a solution with dropdown did not feel right because it would grow fast.
As an improvement for the future - we could link change docs in the versions.html

version 0.20.0-rc1

[0.20.0-rc] Bump version numbers

[0.19.0] Bump version numbers

[0.19.0-rc] Bump version numbers

Revert "Dont call npm before node is available"

This reverts commit 7ecb693.

This breaks Xcode build, see discussion on facebook#5156

[0.18.0-rc] Bump version numbers

[0.17.0] Bump version numbers

Make the NetInfoModule not crash when permission is missing

Depend on 0.16.1 binaries from custom JCenter repo

Only needed until the artifacts propagate to Maven Central.