Problem: migration tests curl times out on CI

Solution: provide option to skip tests with env variable
set in .travis.yml

Rel: ethereumproject#245

.travis.yml

@@ -20,7 +20,7 @@ script:
 - sed -E 's/v([[:digit:]]+\.[[:digit:]]+)\.[[:digit:]]-([[:digit:]]+).+/v\1.\2/' version.txt > version-only.txt
 - sed -E 's/v([[:digit:]]+\.[[:digit:]]+)\.[[:digit:]]-([[:digit:]]+).+/v\1.x/' version.txt > version-base.txt
 - go build -ldflags "-X main.Version="`cat version-app.txt` ./cmd/geth
-- bats cmd/geth
+- env SKIP_MIGRATION_TESTS=true bats cmd/geth
 - zip geth-classic-$TRAVIS_OS_NAME-$(cat version-app.txt).zip geth
 - sed -E "s/APPVERSION/$(cat version-base.txt)/" bintray.tpl.json > bintray.json
 

cmd/geth/migrate_datadir.bats

@@ -5,6 +5,16 @@
 
 setup() {
 
+	# Optionally skip migration tests.
+	# These depend on curl download various geth versions, which sometimes times out
+	# in the CI env. If changes are made to or around migration-related code, this 
+	# env variable should NOT be set. However, if these tests have run and passed
+	# consistently multiple times in the CI environment, it may be considered safe
+	# to skip them to avoid CI false negatives.
+	if [ "$SKIP_MIGRATION_TESTS" ]; then
+		skip "skipping 3.4 migration change tests for CI environment"
+	fi
+
 	# A temporary place to hold current test datadir.
 	DATA_DIR=`mktemp -d`