Use lerna fixed versioning (#209)

Reference: https://github.com/lerna/lerna#fixedlocked-mode-default

1. Currently the SDK create a new tag for each new version each subpackage. This results in having too many tags to maintain.
2. Using independent versioning also adds some unnecessary complexity over the release system.
3. Most popular monorepos use fixed versioning, like Lerna, Babel

A tag for v1.1.1 will be created pointing to this release,
for the last published version that was 86ad0b6

See: aws-sdk-js-v3 switched to fixed versioning with their release candidate in aws/aws-sdk-js-v3#1248
And aws/aws-encryption-sdk-javascript#523

lerna.json

@@ -1,6 +1,4 @@
 {
-  "packages": [
-    "packages/*"
-  ],
-  "version": "independent"
+  "packages": ["packages/*"],
+  "version": "1.1.1"
 }