Preserve the version constraint when running upgrade and upgrade-interactive

[armandabric]

Do you want to request a feature or report a bug?
Bug

What is the current behavior?
When using yarn upgrade-interactive the used version constraint ~ are changed to ^.

# Before `yarn upgrade-interactive`
-    "postcss-reporter": "~2.0.0",
-    "react": "~15.3.1",
-    "react-dom": "~15.3.1",
-    "react-element-to-jsx-string": "~4.2.0",

# After `yarn upgrade-interactive`
+    "postcss-reporter": "^3.0.0",
+    "react": "^15.4.1",
+    "react-dom": "^15.4.1",
+    "react-element-to-jsx-string": "^5.0.5",

What is the expected behavior?
When I upgrade my dependencies I expect that the version constraint are not changed.

Please mention your node.js, yarn and operating system version.
Yarn: 0.18.1