Corrected syntax error (in rubinius) due to no whitespace after '+'

The current version of nickel doesn't work at all in rubinius, due to a syntax error. This pull request corrects that. The cause was that rubinius can't parse + without being surrounded by whitespace.

Here's the full error:

  A syntax error has occurred:
      expecting keyword_do or '{' or '('
      near line /Users/iain/Dropbox/Programs/OSS/nickel/lib/nickel/query.rb:684, column 75

  Code:
            (ZDate.months_of_year.index(m1) + 1).to_s + '/' + m2 + '/' + m3 +' through ' + (ZDate.months_of_year.index(m1) + 1).to_s + '/' + m4 + '/' + m3