Revert "Update VIM plugins."

This reverts commit c28ed7fcce55bad5d60fde1151fcbdc4196ee75c.
Revert "Use murmur airline theme."

This reverts commit 5344a3df1a3a0266fb0ea764ad79eb76ea1032e5.
Revert "Force 256 color mode."

This reverts commit 792a85c778795bdd1c67577b5a420233640343bf.