No longer call #chomp on the end of value duration

According to the ruby documentation, #chomp removes new line characters
such as \r\n or \n from the end of a string.  I don't see any reason to
expect these to be in the duration attribute, and it causes this error
when duration is not present:

NoMethodError: undefined method `chomp' for nil:NilClass
Merge pull request #22 from danielberkompas/fix_zestimateable_chomp

No longer call #chomp on the end of value duration