Update gem and ruby versions to latest stable versions.

[berkes]

Remove EOL ruby versions from travis: don't test unsupported versions. Add recent ruby versions: test current shipped and default versions.

[berkes]

Attempt to fix #177, #178 and #156

Before we can move forward and bump Sqlite to a recent version, the basic dependencies need to be updated to recent versions.

• Ruby: only support alive ruby versions, no EOL rubies.
• Rake, Bundler: recent versions.

[categulario]

amazing job, hope it gets merged

[samg]

Thanks!