Template cleanup travis

[bsipocz]

Some travis matrix cleanup, I think all uncontroversial.

• do not install unneeded packages
• do use stages
• do not use astropy-ci-extras
• no need for sudo any more

[pllim]

xref #279 , #377 , #380 , #392 , #402 , #406

[bsipocz]

I go ahead and merge this, as it should fix the failing build for the rendered branch, too.