Add Travis check for single parameter constructors not marked "explicit"

src/index/blockfilterindex.cpp

@@ -53,7 +53,7 @@ struct DBHeightKey {
     int height;
 
     DBHeightKey() : height(0) {}
-    DBHeightKey(int height_in) : height(height_in) {}
+    explicit DBHeightKey(int height_in) : height(height_in) {}
 
     template<typename Stream>
     void Serialize(Stream& s) const
@@ -76,7 +76,7 @@ struct DBHeightKey {
 struct DBHashKey {
     uint256 hash;
 
-    DBHashKey(const uint256& hash_in) : hash(hash_in) {}
+    explicit DBHashKey(const uint256& hash_in) : hash(hash_in) {}
 
     ADD_SERIALIZE_METHODS;
 

src/interfaces/chain.cpp

@@ -205,7 +205,7 @@ public:
 class RpcHandlerImpl : public Handler
 {
 public:
-    RpcHandlerImpl(const CRPCCommand& command) : m_command(command), m_wrapped_command(&command)
+    explicit RpcHandlerImpl(const CRPCCommand& command) : m_command(command), m_wrapped_command(&command)
     {
         m_command.actor = [this](const JSONRPCRequest& request, UniValue& result, bool last_handler) {
             if (!m_wrapped_command) return false;

src/rpc/util.h

@@ -221,7 +221,7 @@ struct RPCResults {
 
 struct RPCExamples {
     const std::string m_examples;
-    RPCExamples(
+    explicit RPCExamples(
         std::string examples)
         : m_examples(std::move(examples))
     {