When waiting for coverage summary file, create a blank file to watch

[markis]

As the title says, this will create a blank file if the coverage summary file doesn't exist at run time. Most of Jest-Ratchet needs to wait until this file exists and is up to date before it can proceed.

[coveralls]

Coverage increased (+0.3%) to 95.122% when pulling c32f38a on no-file-watch into 256bd51 on master.

[markis]

This fixes #18