Add Solidity Linter to Travis

.soliumrc.json

@@ -0,0 +1,16 @@
+{
+  "extends": "solium:recommended",
+  "plugins": [
+    "security"
+  ],
+  "rules": {
+    "quotes": [
+      "error",
+      "double"
+    ],
+    "indentation": [
+      "error",
+      4
+    ]
+  }
+}

.travis.yml

@@ -52,7 +52,7 @@ matrix:
       install: true # Skip install go packages.
 
       script:
-        # Ensure everything builds
+        # Ensure everything builds.
         - |
           bazel \
             --bazelrc=.travis-bazelrc \
@@ -68,11 +68,24 @@ matrix:
         # Check that BUILD files are formatted correctly.
         - ./check_gazelle.sh
 
-        # Check that target visibility is correct..
+        # Check that target visibility is correct.
         - ./check_visibility.sh
 
         # Shutdown must be last.
-        - bazel shutdown 
-          
+        - bazel shutdown
+
       notifications:
         email: false
+
+    - language: node_js
+      os: linux
+      env:
+        - solidity
+      node_js:
+        - "lts/*"
+      before_install:
+        - npm install -g solium
+      install: true # Skip npm install.
+      script:
+        # Check solidity linter.
+        - solium -d contracts/

contracts/validator_registration.sol

@@ -25,8 +25,14 @@ contract ValidatorRegistration {
         )
         public payable
         {
-        require(msg.value == VALIDATOR_DEPOSIT);
-        require(!usedPubkey[_pubkey]);
+        require(
+            msg.value == VALIDATOR_DEPOSIT,
+            "Incorrect validator deposit"
+        );
+        require(
+            !usedPubkey[_pubkey],
+            "Public key already used"
+        );
 
         usedPubkey[_pubkey] = true;