0.3.8 release

Now that all the important changes/fixes seem to be already merged, I think it is a right time to consider making a new (0.3.8) release of torbrowser-launcher. :-)

@boklm What do you think?

@intrigeri Please confirm that the AppArmor rules added since 0.3.7 work correctly in Debian/Ubuntu. Thanks!

P.S. Do not forget these steps when making the new release: #13 (comment 2983781)