feat(install): omitting the tag now fetches the latest semver tag

[bacongobbler]

Given that you have the following bundles:

$ duffle bundle list
NAME            VERSION DIGEST                                          SIGNED?
helloworld      0.2.0   1d613da00b103a54cbe58d033904b1b9df877c2f        true
helloworld      0.1.0   824475e97a798263d3edda6bd23a58057860d17f        true

duffle bundle install test helloworld will now install v0.2.0.

Note: this now puts a constraint upon duffle where storing non-semver compatible versions in the local store is considered invalid. This was already the case before with duffle build:

$ duffle build .\examples\helloworld\
Error: cannot prepare build: Invalid Semantic Version

This can be worked around with the -f flag, however.

Signed-off-by: Matthew Fisher [email protected]

[bacongobbler]

Closes #551

[vdice]

LGTM, though one unit test messaging question.