Check for the existence of :Make, not :Makeshift

When vimscript#4278 is installed alongside vim-rails, calling `:make` throws the following error:

```
Error detected while processing function <SNR>84_Rake..<SNR>84_make:
line    2:
E488: Trailing characters: Make test:units TEST=test/unit/product_test.rb
```

Issue logged here: https://github.com/johnsyweb/vim-makeshift/issues/2