Carthage: unpin OpenSSL 1.0.2

Previously, krzyzanowskim/OpenSSL 1.0.2.18 was broken and we were not
even able to revert to 1.0.2.17 because Carthage mandates three-part
semver-compatible versions of dependencies (though packages themselves
can have more version parts). Since then Marcin has pushed an update
to 1.0.2.18 which fixes the original linkage issue. It also introduces
new versioning scheme (1.0.218) which allows reverting, just in case.

We can go back to using "~> 1.0.2" as our dependency spec. Remove the
workaround and update the lock file as well.

You can read the whole backstory here:

  cossacklabs#483
  cossacklabs#484

and in other pull requests and issues linked from there.

Cartfile

@@ -1,9 +1 @@
-# temporary use our fork due to errors in krzyzanowskim/OpenSSL 1.0.2.18
-# https://github.com/krzyzanowskim/OpenSSL/issues/64
-# https://github.com/krzyzanowskim/OpenSSL/issues/63
-# hash of 1.0.2.17 tag
-github "krzyzanowskim/OpenSSL" "990bd88"
-
-# broken tag is 1.0.2.18
-# github "krzyzanowskim/OpenSSL" ~> 1.0.2
-
+github "krzyzanowskim/OpenSSL" ~> 1.0.2

Cartfile.resolved

@@ -1 +1 @@
-github "krzyzanowskim/OpenSSL" "990bd88219da80d7a77289aeae245b3eb400d834"
+github "krzyzanowskim/OpenSSL" "1.0.218"