Bump bson to 0.7.0 (#207)

Author: anderspitman

README.md

@@ -22,7 +22,7 @@ The driver is available on crates.io. To use the MongoDB driver in your code, ad
 
 ```
 [dependencies]
-bson = "0.6.1"
+bson = "0.7.0"
 mongodb = "0.2.6"
 ```
 
@@ -149,4 +149,4 @@ Run tests like a regular Rust program:
 
 ```
 cargo test --verbose
-```
+```