Fix the lint checks in Travis

[cmihai]

Currently, the lint checks in Travis are executed, but don't fail the build. Since nobody really looks at the lint output, it's as if we didn't have a linter at all. The following checks in test/lint/ need to be fixed and re-enabled (or removed):

• [x] Git subtree checks for src/crypto/ctaes, src/secp256k1, src/univalue and src/leveldb
• [x] lint-circular-dependencies.sh
• [x] lint-filenames.sh
• [x] lint-format-strings.sh
• [x] lint-include-guards.sh (#1123)
• [x] lint-includes.sh
• [x] lint-locale-dependence.sh
• [x] lint-logs.sh
• [x] lint-python.sh
• [x] lint-python-shebang.sh
• [x] lint-python-utf8-encoding.sh
• [x] lint-shell-locale.sh
• [ ] lint-shell.sh
• [ ] lint-tests.sh
• [x] lint-whitespace.sh