Relax dependency versions (#5)

Author: chingor13

composer.json

@@ -4,8 +4,8 @@
     "type": "library",
     "require": {
         "php": ">=5.6",
-        "opencensus/opencensus": "^0.4",
-        "google/cloud-trace": "^0.4"
+        "opencensus/opencensus": "~0.4",
+        "google/cloud-trace": "~0.4"
     },
     "require-dev": {
         "phpunit/phpunit": "^6.0",