el-get-byte-compile-file: delete old compiled file

This should prevent the discrepancy that occurred before when
compilation failed.
recipe: racket-mode
recipe: keep-buffers
elisp-slime-nav: use build-in enable function instead of defining one
add recipe for PHP Code Sniffer with Flymake
Merge pull request #1984 from martinburchell/master

add recipe for PHP Code Sniffer with Flymake
Merge pull request #1983 from deftsp/pin/dev

elisp-slime-nav: use build-in enable function instead of defining one
recipe for the karma package
Merge pull request #1985 from tonini/karma-recipe

recipe for the karma package
Delete elc before self-update

* el-get.el (el-get-self-update): Delete elc files before updating
  el-get.