Possible bug!! Replacing str in dataframe delete entire number
```
df.head()
    OSymbol
0   .GGP:150918:C:18.0
1   .GGP:150918:C:19.0
2   .GGP:150918:C:20.0
3   .GGP:150918:C:21.0
4   .GGP:150918:C:22.0

df['OSymbol'].str.replace(".0", "")

0           .GGP:1918:C:18
1           .GGP:1918:C:19
2             .GGP:1918:C:
3           .GGP:1918:C:21
4           .GGP:1918:C:22
```

Look at 20 strike... should be 20.0 > 20 , instead all of 20.0 is removed...

Pandas version 0.16.2 
