Bug 1386754 - Deprecate 3DES on nightly builds r=dveditz

3DES usage is well below 1% [1]. A recent TLS Canary run with
"security.ssl3.rsa_des_ede3_sha" disabled showed 825 of 500,000 sites being
affected by deprecating 3DES [2]. This starts the deprecation by disabling the
3DES cipher suite preference in nightly builds.

[1] https://mzl.la/2uNt0BP
[2] https://tlscanary.mozilla.org/runs/2017-08-01-17-07-49/

MozReview-Commit-ID: C9cobeOqGLj

--HG--
extra : rebase_source : 07b433efff1b5e3812a0dd54569bbe4198981328

security/manager/ssl/security-prefs.js

@@ -28,7 +28,12 @@ pref("security.ssl3.dhe_rsa_aes_128_sha", true);
 pref("security.ssl3.dhe_rsa_aes_256_sha", true);
 pref("security.ssl3.rsa_aes_128_sha", true);
 pref("security.ssl3.rsa_aes_256_sha", true);
+// Deprecate 3DES on nightly builds, Bug 1386754
+#ifdef RELEASE_OR_BETA
 pref("security.ssl3.rsa_des_ede3_sha", true);
+#else
+pref("security.ssl3.rsa_des_ede3_sha", false);
+#endif
 
 pref("security.content.signature.root_hash",
      "97:E8:BA:9C:F1:2F:B3:DE:53:CC:42:A4:E6:57:7E:D6:4D:F4:93:C2:47:B4:14:FE:A0:36:81:8D:38:23:56:0E");