evm does not report version

[raxod502]

% gem install evm
Fetching: evm-0.9.1.gem (100%)
Successfully installed evm-0.9.1
Parsing documentation for evm-0.9.1
Installing ri documentation for evm-0.9.1
Done installing documentation for evm after 0 seconds
1 gem installed
% evm --version
evm: version unknown

System: macOS 10.11.6
RubyGems: 2.7.6 installed via Homebrew

[sambrightman]

This (automatic) functionality of OptionParser seem to be quite under-documented (and --version isn't an option listed in --help anyway). A PR that a) avoids duplicating the version string from the gemspec; b) works for all supported installation methods would be welcome.