Improve prepack script (#1747)

* improve prepack script

* remove .npmignore

* make prepack use pkg.files

* fix linter errors

(cherry picked from commit cc19ccfdb3)

scripts/compile.sh

@@ -0,0 +1,10 @@
+#!/usr/bin/env sh
+
+if [ "$SOLC_NIGHTLY" = true ]; then
+  docker pull ethereum/solc:nightly
+fi
+
+# Necessary to avoid an error in Truffle
+rm -rf build/contracts
+
+truffle compile