Strip leading 'v' from git tags if present.

[wadells]

As discussed at:

golang/go#32945
https://semver.org/#is-v123-a-semantic-version

Go and semver disagree about handling of a leading 'v' on version
strings. This patch allows robotest to play nicely with both.

I discovered this when I created the v2.0.0-alpha.1 tag in git.

Contributes to #200.

Testing Done

Before:

walt@work:~/git/robotest$ git describe --tags
v2.0.0-alpha.1
walt@work:~/git/robotest$ make version       
version metadata saved to version.go
Robotest Version: v2.0.0-alpha.1

After:

walt@work:~/git/robotest$ git describe --tags        
v2.0.0-alpha.1-1-g4ac1444
walt@work:~/git/robotest$ make version       
version metadata saved to version.go
Robotest Version: 2.0.0-alpha.1.1+4ac14441

With a non-v-prefixed tag:

walt@work:~/git/robotest$ git tag 3.fake.version
walt@work:~/git/robotest$ make version
version metadata saved to version.go
Robotest Version: 3.fake.version
walt@work:~/git/robotest$ git tag -d 3.fake.version 
Deleted tag '3.fake.version' (was 4ac1444)