feat: Retrieve version gitHead from git tags and unshallow the repo if necessary

Add several fixes and improvements in the identification of the last release gitHead:
- If there is no last release, unshallow the repo in order to retrieve all existing commits
- If git head is not present in last release, try to retrieve it from git tag with format ‘v\<version\>’ or ‘\<version\>’
- If the last release git head cannot be determined and  found in commit history, unshallow the repo and try again
- Throw a ENOGITHEAD error if the gitHead for the last release cannot be found in the npm metadata nor in the git tags, preventing to make release based on the all the commits in the repo as before
- Add integration test for the scenario with a packed repo from which `npm republish` fails to read the git head

Fix #447, Fix #393, Fix #280, Fix #276

.travis.yml

@@ -13,10 +13,6 @@ branches:
     - caribou
     - /^greenkeeper.*$/
 
-# Retrieve 999 commits (default is 50) so semantic-release can analyze all commits when there is more than 50 on a PR
-git:
-  depth: 999
-
 # Retry install on fail to avoid failing a build on network/disk/external errors
 install:
   - travis_retry npm install