www/firefox: update to 61.0

• Drop DTRACE option, as it no longer builds https://bugzilla.mozilla.org/show_bug.cgi?id=1448659 https://bugzilla.mozilla.org/show_bug.cgi?id=702179#c33
• Stop using system hunspell per https://bugzilla.mozilla.org/show_bug.cgi?id=1460600

Changes: https://www.mozilla.org/firefox/61.0/releasenotes/
PR: 227850
Security: cd81806c-26e7-4d4a-8425-02724a2f48af
MFH: 2018Q2
Differential Revision: https://reviews.freebsd.org/D15227