Update travis with slither package

scripts/travis_install.sh

@@ -1,5 +1,5 @@
 #!/usr/bin/env bash
-pip install -r requirements.txt
+python setup.py install
 
 function install_solc {
     sudo wget -O /usr/bin/solc https://github.com/ethereum/solidity/releases/download/v0.4.24/solc-static-linux

scripts/travis_test.sh

@@ -1,11 +1,11 @@
 #!/usr/bin/env bash
 
-./slither.py examples/bugs/uninitialized.sol --disable-solc-warnings
+slither examples/bugs/uninitialized.sol --disable-solc-warnings
 if [ $? -ne 1 ]; then
     exit 1
 fi
 
-./slither.py examples/bugs/backdoor.sol --disable-solc-warnings
+slither examples/bugs/backdoor.sol --disable-solc-warnings
 if [ $? -ne 1 ]; then
     exit 1
 fi