Skip doctests for README.md on 1.18.0

rustdoc --test with 1.18.0 is running tests twice, which leads to a race
condition where the second test run might fail if the first test run
deletes the shared record. See rust-lang/rust#42726.

Author: jhecking

.travis.yml

@@ -20,8 +20,8 @@ before_script:
 script:
   - rm -rf target/debug/deps/*aerospike*
   - cargo build
-  - cargo test
-  - rustdoc -L target/debug/deps/ --test README.md
+  - cargo test --verbose
+  - [ $TRAVIS_RUST_VERSION != "1.18.0" ] && rustdoc -L target/debug/deps/ --test README.md
 after_script:
 - tail -n +1 aerospike-server/instance?/var/log/aerospike.log
 notifications: