Remove redundant solidus_core dev dependency

[brchristian]

When I was testing the 0.5.1 release on my local machine, running bundle install showed the following warning:

Your Gemfile lists the gem solidus_core (>= 0) more than once.
You should probably keep only one of them.
Remove any duplicate entries and specify the gem only once.
While it's not a problem now, it could cause errors if you change the version of one of them later.

It looks like if we already require solidus_core in the Gemfile here:

solidus_support/Gemfile

Line 7 in 1683eb6

gem 'solidus_core', github: 'solidusio/solidus', branch: branch

Then we don't need to also include it as a development dependency in solidus_support.gemspec.

[spaghetticode]

@brchristian thank you 👍