Trim quotes on value

I want to write `.env` file with quotes. So it is nice heroku-config trims those quotes.

```ruby
# REPL
hash = {}
line = %(KEY='abcde4567abcd')

if line =~ /\A([A-Za-z0-9_]+)=(.*)\z/ then hash[$1] = $2.gsub(/\A(?:'|")(.*)(?:'|")\z/) { $1 } end
hash #=> {"KEY"=>"abcde4567abcd"}

line = %(KEY='abcde4'567abcd')
if line =~ /\A([A-Za-z0-9_]+)=(.*)\z/ then hash[$1] = $2.gsub(/\A(?:'|")(.*)(?:'|")\z/) { $1 } end
hash #=> {"KEY"=>"abcde4'567abcd"}
```

Thanks for nice gem! :gem: