Merge remote-tracking branch 'teor/ticket27252-032' into maint-0.3.2

changes/ticket27252

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+  o Minor features (continuous integration):
+    - Skip gcc on OSX in Travis CI, it's rarely used.
+      Skip a duplicate hardening-off build in Travis on Tor 0.2.9.
+      Skip gcc on Linux with default settings, because all the non-default
+      builds use gcc on Linux.
+      Implements ticket 27252.

changes/ticket27252-032

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+  o Minor features (continuous integration):
+    - Only run one online rust build in Travis, to reduce network errors.
+      Skip offline rust builds on Travis for Linux gcc, because they're
+      redundant.
+      Implements ticket 27252.