Update GUESS_FIELDS option in .reviewboardrc to support current version of RBTools.

According to the instruction of using Review Board, we can create a review request by running the following command:

$ rbt post

However, if we use current version of RBTools (0.7.6), above command doesn't work because the values accepted to the option "--guess-fields" was updated since 0.6.

$ rbt --version
RBTools 0.7.6
$ rbt post
ERROR: Invalid value "true" for argument "--guess-fields"

It is better to update the value of GUESS_FIELDS option in .reviewboardrc to avoid this error.