Peter Bengtsson: hashin 0.5.0 bug fix

Thank you @davehunt for finding a ugly bug in in hashin.

The bug happened when you tried to add a package that had a similar name to a package that was already in your requirements.txt. By similar name, it was if the package name was inside another name. E.g. 'selenium==' in 'pytest-selenium=='.

Here's the fix.

So make sure you upgrade to version >0.5.