Fix error emitted from nuclide-diff-gutters

Summary:
Turns out if you have an observable with `.first()` then completes before it emits a value, it will throw an error. This was causing the observable to throw in some cases and cause the feature to stop working. `.take(1)` doesn't throw

Repro'd by adding a `.catch` to the end of the observable.  Opening some files, checking out a new revision, then using 'Close other tabs'.

Reviewed By: evangrayk

Differential Revision: D9813359

fbshipit-source-id: 71f37900122be68b08031309066635636732ed30

pkg/nuclide-diff-gutters/lib/main.js

@@ -119,7 +119,7 @@ class Activation {
                 ),
               )
               .filter(isVisible => isVisible)
-              .first()
+              .take(1)
               .flatMap(() => {
                 const bufferPath = nullthrows(textEditor.getPath());
                 // TODO (tjfryan): do something to handle generated files