When is 我 pronounced ě?
As others have explained, ě is currently used in certain nonstandard dialects. You might be interested to know that ě is the expected pronunciation based on the regular sound changes that connect Mandarin to the Middle Chinese pronunciations recorded in rhyming dictionaries. Compare these words: 

我: Mand. wǒ, MC nga, rising tone,
   餓: Mand. è, MC nga, departing tone,
   蛾: Mand. é, MC nga, level tone,

which had the same initial and vowel but different tones. The pronunciation wǒ is not expected purely from the historical evidence of old dictionaries. I'm not certain where it came from, but it could have been influenced by words such as 

吾: Mand. wú, MC ngu, level tone.

What I've written so far isn't enough to prove anything, but hopefully it provides more context. Another interesting question might be "Why is 我 pronounced wǒ?" It appears to be a slang usage that became standardized because of the political and economic power of the regions it's used in.