Remove gi and giv aliases

Often mistyping things like `gi tst` causes `gi` alias to trigger the install of
the `tst` gem. This is quite annoying and too close to `git` command which we
run much more often than `gem install`. With bundler we should almost never run
`gem install`.
Improve zsh history

* "to the $HISTFILE incrementally (as soon as they are entered), rather than
  waiting until the shell exits"
* replace any duplicate in the history, not just the previous duplicate: allows
  for a cleaner history
* `.zhistory` is more idiomatic than `.zsh_history`
Add syntax highliting for slim via vim-slim

We are using slim more and more on projects.
Remove RSpec -- profile option

* This option adds a lot of noise to test output
* Extra noise often means scrolling
* The profile includes red, which makes me think tests fail
* Profiling isn't useful when running one or a few tests
* I'm usually not in the mindset of profile tests, so I ignore it