fix(magic-value): make incrementing magic value a manual step (#641)

We should not change the hash used to identify coverage reports unless
an incompatible change is actually introduced. In practice, we should
try to make sure changes are backwards compatible.

packages/istanbul-lib-instrument/src/constants.js

@@ -1,12 +1,14 @@
 const { createHash } = require('crypto');
-const { major } = require('semver');
-const { name, version } = require('../package.json');
+const { name } = require('../package.json');
+// TODO: increment this version if there are schema changes
+// that are not backwards compatible:
+const VERSION = '4';
 
 const SHA = 'sha1';
 module.exports = {
     SHA,
     MAGIC_KEY: '_coverageSchema',
     MAGIC_VALUE: createHash(SHA)
-        .update(name + '@' + major(version))
+        .update(name + '@' + VERSION)
         .digest('hex')
 };