Replace main() with unit test to make Travis CI work

src/lib.rs

@@ -0,0 +1,14 @@
+extern crate base64;
+extern crate ring;
+
+mod string_encryption_key_based_symmetric;
+
+#[cfg(test)]
+mod tests {
+    #[test]
+    fn test_tring_encryption_key_based_symmetric() {
+        // This test runs all the examples to ensure that they compile and work.
+        ::string_encryption_key_based_symmetric::main();
+        // TODO check program output contains a line stating that the encryption and decryption worked
+    }
+}