GH-5: GitGutter() function not found during `vih`

The GitGutter plugin's API has changed.  It has replaced the GitGutter() 
function with a :GitGutter command that updates signs in current buffer. As a
result, I get the following error when typing `vih` in normal mode:

    Error detected while processing function
98..<SNR>140_select_function_wrapper..textobj#gitgutter#select_i:
   line    1:
   E117: Unknown function: GitGutter
   Press ENTER or type command to continue

This patch updates our use of GitGutter() to :GitGutter to fix the bug.