Using `>0.8.0 <=0.9.0` instead of `>0.7.0 <=0.9.0`

[adityakode]

Using >0.8.0 <=0.9.0 instead of >0.7.0 <=0.9.0 will have some benefits ,

• Safemath by default from 0.8.0
• Low-level inliner from 0.8.2, leads to cheaper runtime gas. Also , It would be easier for testing .

Please assign this to me under GSSoC. Thank You!