Fix solc-nightly job (#1732)

* update truffle to 5.0.14

* fix setup to test with solc-nightly

* switch to npx in script/test.sh

* please the linter

* rename build to prepack

* move download of nightly build to a compile script

* make compile script executable

scripts/compile.sh

@@ -0,0 +1,7 @@
+#!/usr/bin/env sh
+
+if [ "$SOLC_NIGHTLY" = true ]; then
+  docker pull ethereum/solc:nightly
+fi
+
+truffle compile