Fixed 'before-insert' hook

Fixed 'before-insert' hook: now checks if language is PHP before assigning env.code to env.backupCode.
Only "language-php" defines env.backupCode: without this condition, env.Code would be undefined for any other language than PHP after this hook is called. It would then lead to wrong behaviours (eg. Line Numbers plugin would not show lines).
Merge pull request #343 from GuiTeK/fix-prism-php-before-insert-hook

Fixed 'before-insert' hook