Tweak gemspec to be less specific for certain gems

Lessens the restrictions on fog-core and google-api-client. Trying to slowly remove the warnings you get when you run `rake build` such as:

```
WARNING:  pessimistic dependency on shindo (~> 0.3.4, development) may be overly strict
  if shindo is semantically versioned, use:
    add_development_dependency 'shindo', '~> 0.3', '>= 0.3.4'
```

bundle update diff:
```diff
$ diff -u old Gemfile.lock
--- old	2014-03-14 15:11:59.000000000 -0700
+++ Gemfile.lock	2014-03-15 03:45:13.000000000 -0700
@@ -37,10 +37,10 @@
       term-ansicolor
       thor
     docile (1.1.3)
-    excon (0.31.0)
+    excon (0.32.1)
     extlib (0.9.16)
-    faraday (0.8.9)
-      multipart-post (~> 1.2.0)
+    faraday (0.9.0)
+      multipart-post (>= 1.2, < 3)
     fast-stemmer (1.0.2)
     ffi (1.9.3)
     fission (0.5.0)
@@ -48,9 +48,9 @@
     fog-brightbox (0.0.1)
       fog-core
       fog-json
-    fog-core (1.21.0)
+    fog-core (1.21.1)
       builder
-      excon (~> 0.31.0)
+      excon (~> 0.32)
       formatador (~> 0.2.0)
       mime-types
       net-scp (~> 1.1)
@@ -58,15 +58,16 @@
     fog-json (1.0.0)
       multi_json (~> 1.0)
     formatador (0.2.4)
-    google-api-client (0.6.4)
+    google-api-client (0.7.1)
       addressable (>= 2.3.2)
       autoparse (>= 0.3.3)
       extlib (>= 0.9.15)
-      faraday (~> 0.8.4)
+      faraday (>= 0.9.0)
       jwt (>= 0.1.5)
       launchy (>= 2.1.1)
       multi_json (>= 1.0.0)
-      signet (~> 0.4.5)
+      retriable (>= 1.4)
+      signet (>= 0.5.0)
       uuidtools (>= 2.1.0)
     highline (1.6.21)
     jekyll (1.4.3)
@@ -93,9 +94,9 @@
     method_source (0.8.2)
     mime-types (2.2)
     mini_portile (0.5.2)
-    minitest (5.3.0)
+    minitest (5.3.1)
     multi_json (1.9.0)
-    multipart-post (1.2.0)
+    multipart-post (2.0.0)
     net-scp (1.1.2)
       net-ssh (>= 2.6.5)
     net-ssh (2.8.0)
@@ -127,12 +128,13 @@
     redcarpet (2.3.0)
     rest-client (1.6.7)
       mime-types (>= 1.16)
+    retriable (1.4.1)
     safe_yaml (0.9.7)
     shindo (0.3.8)
       formatador (>= 0.1.1)
-    signet (0.4.5)
+    signet (0.5.0)
       addressable (>= 2.2.3)
-      faraday (~> 0.8.1)
+      faraday (>= 0.9.0.rc5)
       jwt (>= 0.1.5)
       multi_json (>= 1.0.0)
     simplecov (0.8.2)
```